Abstract:

In modelling vegetated flows and the associated processes, the flow resistance of riparian vegetation growing along riverbanks and floodplains has received substantial attention. This has led to the development of new resistance or drag force models and the concomitant proposal of several alternative parameters for the resistance estimation. However, the reliability of these models, the effect of reconfiguration (i.e. streamlining and bending of the specimens), and the suitability of different plant parameterizations for woody vegetation of different scales and vegetation densities have remained unclear. Moreover, retrieval of the vegetation properties is laborious with conventional methods. For data retrieval, terrestrial laser scanning (TLS) provides an advanced method which enables high-resolution surveying of floodplain topography and vegetation properties.

The aim of this dissertation was 1) to determine the main vegetation properties impacting the drag exerted by woody foliated vegetation of different scales and of varying horizontal and vertical densities, and 2) to assess the reliability of TLS as a method for obtaining herbaceous and woody vegetation properties and floodplain ground level for different seasons. For these purposes, drag force measurements were conducted in a flume in arrays and in a towing tank for natural trees of different sizes. In the flume, the plant areas of the individual specimens and the spacing of the vegetation in an array were both altered. In a towing tank, four species of 0.9-3.4 m in height were experimented together with detailed characterization of tree properties. TLS campaigns were conducted in connection with the towing tank experiments and in a field floodplain site.

The stem, leaf, and total areas proved to be reliable vegetation properties for the drag estimation despite the variability in tree height. The commonly applied frontal projected area is not an adequate parameter, as the leaves behind the frontal area still exert drag. The parameterization of the reconfiguration appeared to be independent of the tree size. Consequently, a drag force formula was derived for foliated trees with the total plant area and for the defoliated ones with the stem area as a characteristic reference area. For obtaining plant properties in the field, a new concept was proposed for reach scale analyses based on upscaling of the relationships between TLS data and manually measured plant areas derived for small subareas. Overall, these results of the physically-based modelling of drag forces with the TLS-based plant characterization provide useful knowledge for hydro-environmental modelling purposes.Jokivarsien kasvillisuudella on merkittävä rooli tulvanhallinnassa, eroosiosuojana sekä kiintoaineen ja ravinteiden pidättäjänä. Vedenpinnankorkeuksien, virtausnopeuksien ja aineiden kulkeutumisen arvioimisen luotettavuuden parantamiseksi tarvitaan malleja, jotka käyttävät parametreina kasvien fysikaalisia ominaisuuksia ja ottavat huomioon kasvien muodonmuutoksen virtaustilanteessa (virtaviivaistuminen ja taipuminen). Puiden oksilla tehdyt kourukokeet ovat kuitenkin olleet vaikeasti skaalattavia luonnonpuille ja -pensaille. Modernit kaukokartoitusmenetelmät, kuten maalaserkeilaus (TLS), puolestaan mahdollistavat erittäin tarkan 3D-paikkatiedon keräämisen kasvillisuudesta ja sen ominaisuuksista maastossa.